The Hedgehog (Hh) pathway has been implicated in many developmental processes, including organogenesis in most animals (Ingham, P. W., McMahon, A. P. Genes and Dev. 15, 3059-3087, 2001; Frank-Kamenetsky, M., et al. J. Biol. 10, 1-19, 2002). First identified in Drosophila in 1980, three mammalian homologues of Hh proteins, Sonic Hedgehog (Shh), Desert Hedgehog, (Dhh), and Indian Hedgehog (Ihh), are all key regulators of anterior/posterior patterning in limb development, the induction of polarity in the central nervous system, and the differentiation of numerous cell types (Nusslein-Volhard, C., Wieschaus, E. Nature 287, 795-801, 1980); Weitzman, J. B. J. Biol. 7, 1-5, 2002; Pepinsky, R. B. et al. J. Biol. Chem. 275, 10995-11001, 2000; Stecca, B., Altaba, A. R. J. Biol. 9, 1-4, 2002). Shh is the most widely characterized of the Hh homologues and is essential for proper embryonic development. The Shh pathway involves the auto-cleavage of full length Shh into an active 20 kD N-terminal fragment (ShhN), which binds to its 7-pass transmembrane receptor, Patched (Ptc1), reversing its inhibitory effect on Smoothened (Smo) (Goetz, J. A., Singh, S., Suber, L. M., Robbins, D. J. J. Biol. Chem. 281, 4087-4093, 2006). One effect of this de-repression is the activation of Gli transcription factors, which regulate the transcription of target genes that include Gli1 and Ptc1.
There have been several reports of both synthetic and natural small-molecule modulators of the Shh signaling pathway, discovered through cell-based phenotypic screens (Rubin, L., de Sauvage, F. J. Nature 5, 1026-1033, 2006; Chen, J. K., Tapaile, J., Young, K. E., Maiti, T., Beachy, P. A. PNAS 99, 14071-14076, 2002). Reported Shh signaling antagonists include teratogenic natural products such as cyclopamine, jervine, AY9944, and tripanol, as well as synthetic molecules such as SANT1 and Cur-6141 (Cooper, M. K., Porter, J. A., Young, K. E., Beachy, P. A. Science 280, 1603-1607, 1998; Tapaile, J., et al. Nature 406, 1005-1009, 2000). There have also been reports of synthetic small-molecule agonists of the Shh pathway, including purmorphamine and Hh-Ag1.2 (Ding, S., Schultz, P. G. Nat. Biotechnol. 7, 833-840, 2004; Wu, X. et al. Chem. Biol. 11, 1229-1238, 2004). The discovery of chemical modulators of Shh signaling provides a potential means to regulate the activity of a pathway that can result in proliferative diseases (e.g., medulloblastoma, basal cell carcinomas, or pancreatic cancer), pulmonary diseases (Coon, D. R. et al. Exp. Mol. Pathol. 80, 119-123, 2006; Stewart, G. A. et al. J. Pathol. 199, 488-495, 2003) and developmental disorders (Wang, B., Fallon, J. F., Beachy, P. A. Cell 100, 423-434, 2000; Borycki, A. G., Mendham, L., Emerson, C. P., Jr. Development 125, 777-790, 1998; Chiang, C. et al. Nature 383, 407-413, 1996; Kayed, H. et al. Pancreas 32, 119-129, 2006; Furukawa, T., Sunumura, M., Hori, A. Cancer Sci., 97, 1-7, 2006; Thayer, S. P. et al. Nature 425, 851-856, 2003).